HOW ANIMALS BREATHE. . AES 
cavity for the heart and lungs, with movable walls (breast- 
bone and ribs) and a diaphragm, or muscular partition, 
separating it from the abdomen.” Inspiration (or filling 
the lungs) and expiration (or emptying the lungs) are both 
accomplished by muscular exertion; the former, by rais- 
ing the ribs and lowering the diaphragm, which enlarge 
the capacity of the chest, and the air rushes in to prevent 
a vacuum; the latter, by the ascent of the diaphragm and 
the descent of the ribs. 
As a rule, the more active and more muscular an ani- 
mal, the greater the demand for oxygen. Thus, warm- 
blooded animals live fast, and their rapidly decaying tis- 
sues call for rapid respiration; while in the cold-blooded 
creatures the waste is comparatively slow. Respiration is 
most active in Birds, and least in water-breathing animals. 
The sluggish Toad respires more slowly than the busy 
Bee, the Mollusk more slowly than the Fish. But respi- 
rations, like beats of the heart, are fewer in large Mam- 
mals than in small ones. An average Man inhales about 
700 cubit feet of air per day. 
Another result of respiration, besides the purification of 
the blood, is the production of heat. The chemical com- 
bination of the oxygen in the air with the carbon in the 
tissues is a true combustion; and, therefore, the more act- 
ive the animal and its breathing, the higher its tempera- 
ture. Lirds and Mammals have a temperature of about 
108° and 100° respectively, and are called warm-blooded. 
Fishes and Reptiles have a lower and more variable tem- 
perature, ranging from 35° to 80°, and are called cold- 
blooded. The Bee is from 3° to 10°, and the Earth-worm 
and Snail from 14° to 2°, higher than the air. The mean 
temperature of the Carp and Toad is 51°; of Man, 98°; 
Dog, 99°; Cat, 101°; Squirrel, 105°; Swallow, 111°. 
